The present invention relates to a method and a device for directly digitizing microwave signals reflected on a filling product surface of a filling product present in a receptacle, and to a filling level measuring device working on the transit time principle and emitting microwave signals.
In simplified terms, microwave signals are generated (in a pulsed or continuous manner) by a transmitting means in a filling level measuring device, and are radiated via an antenna, such as, for example, a horn, rod or microstrip antenna into the direction of the filling product surface to be measured or are guides to the filling product surface by means of a waveguide (rod, cable or also rope). The microwave signals reflected on the filling product surfacexe2x80x94in general also designated as echo signalsxe2x80x94may be fed to a reception circuit and scanned via a receiving means, often identical to the transmitting antenna; or by means of the aforementioned waveguide. The analog values resulting from said scanning are converted into digital values, and are then made available to an evaluation means. From the digital values, the filling level height is determined in the evaluation means.
Reference is, for example, made to the document DE 42 40 491 A1, in which such a measuring device is disclosed in general. For processing the reflected and received microwave signals, various analog circuits or components are present so as to generate a so-called envelope curve, which will then be digitized for determining the filling level height therefrom. The generation of an envelope curve is well known, and, correspondingly, there is no need for further explanations as to this technology (with respect to the generation of the envelope curve, reference is made, for example, to DE 44 07 396 C2 and M. Scholnik xe2x80x9cIntroduction to Radar Systemsxe2x80x9d, 2nd edition, 1980, McGraw-Hill; Peter Devine xe2x80x9cRadar level measurementxe2x80x94the user""s guidexe2x80x9d, VEGA Controls Ltd., 2000, ISBN 0-9538920-0-X, which are hereby included by reference).
In the present technical field, microwave signals are worked with that have a frequency of 1 to 30 GHz or more. Due to these high frequencies, various analog circuits or components may be necessary for determining the exact filling level height from the received microwave signals. In particular, a so-called mixer has inter alia to be used, with the assistance of which a temporal xe2x80x9celongationxe2x80x9d of the envelope curve is carried out, so that the scanning of the envelope curve by means of conventional A/D converter (pulse frequencies of about 25 microseconds) with a sufficient fine scanning screen is possible (for achieving a sufficient precision of the determined filling level height). This means that microwave pulses are in particular transformed into another frequency and time domain by means of the mixer and the associated oscillator. Hereby, pulses are for example temporally elongated after time transformation approximately by a factor of 160,000, whereby a pulse duration of 1-2 nanoseconds before the transformation corresponds to a pulse duration of about 160-320 microseconds after the transformation. These time-elongated pulses are then fed to the A/D converter that scans said signal in a scanning screen of about 25 microseconds. Thus, with a pulse length of 160 microseconds, six scanning values are available per pulse. With these values, accuracies of about 5 mm may be obtained in the determination of the filling level height. It has to be pointed out that the precision of the filling level determination is also dependent on the height of the scanning values per echo pulse, on the gradient of the pulse flanks, and on the accuracy of the A/D converter.
In summary, it has to be stated that devices for determining the filling level height basing on reflected microwaves, in particular microwave pulses, perform an analog processing of the received signals for being able to achieve the desired accuracy in the filling level height determination. Thus, for example, reference is made to the document U.S. Pat. No. 6,087,978 that discloses a pulsed radar-filling level measuring device, wherein the microwave signals are transformed into another frequency and time domain with the assistance of a mixer and an associated oscillator so as to be able to carry out the scanning of the signals and the digitization of the scanning values. In the document U.S. Pat. No. 5,847,567, a filling level measuring device is described that is either configured as a FMCW radar device or as a pulsed radar device. With the configuration as an FMCW radar device, a mixer of the aforementioned kind is present serving for the transformation of the frequency domain, as well as the time domain of the incoming signals. Then, an analog-digital conversion of the signals is performed. With the configuration as a pulsed radar device, a transformation is performed in the above-described manner by means of a timing circuit.
It is believed that the use of analog components or analog switching technique, however, is affected with disadvantages. Thus, with the dimensioning of the circuit, component tolerances and fabrication divergences have to be taken into consideration, and have also to be monitored during the production. Analog components moreover exhibit temperature-dependence, which have to be considered and cause a not inconsiderable test expenditure. It is believed that temperature-dependence and component tolerances in addition result in a decrease of the reproducibility and accuracy of the initial signals, and have to be minimized or compensated by an additional expenditure. It is also believed that a disadvantage has also to be seen in that modifications in the functional mode of such an analog, circuit can only be carried out at a high expenditure. Finally, it is believed that analog circuits also are sensitive to disturbing influences.
A technical problem on which an exemplary embodiment of the present invention is based, consists in providing a method and a device by means of which the number of analog circuit components in devices of the mentioned kind can be reduced, and the above-described disadvantages are therewith at least decreased.
It is believed that this technical problem may be solved according to a first aspect of the invention by an exemplary embodiment of a method for directly digitizing microwave signals reflected on a filling product surface of a filling product present in a receptacle, wherein the microwave signals reflected on the filling product surface are scanned, and the individual scanning values resulting from said scanning, are stored in an analog value storage means, stored analog values having a lower rate as compared to the scanning rate being read out from the analog value storage means and being converted into digital values made available to an evaluation means for determining the filling level height.
According to another exemplary embodiment of the method it is provided that the microwave signals are scanned either unchanged in their frequency or substantially unchanged at a high scanning rate corresponding to this frequency, and that the analog values generated by the scanning process are stored at a storage rate corresponding to the scanning rate in a so-called FISO system, and that furthermore, the analog values stored in said FISO system are read out from said FISO system at a lower read-out rate as compared to the storage rate, and are converted into digital values, and that these digital values in turn are stored in a digital value storage means at a storage rate corresponding to said read-out rate, and that finally, the evaluation means for determining the filling level height, accesses the digital value storage means for reading out the therein stored digital values.
Such circuit arrangements are designated as FISO system (Fast In Slow Out) allowing for a fast read-in of analog valuesxe2x80x94hence at a very high frequency or a high scanning ratexe2x80x94and outputting said values in a slower manner, so that one or several A/D converters of the conventional construction can be connected thereto. FISO systems usable here, are in principle disclosed, for example in the documents U.S. Pat. No. 5,144,525 A or U.S. Pat. No. 5,200,983 A. A Waveform Digitizer Card, which can basically be used here, is distributed under the designation xe2x80x9cCompuscope 85Gxe2x80x9d by the company GAGE which is affiliated with Tektronix Technology Company.
According to another exemplary embodiment of a method according to the above-given explanations which is believed to be advantageous, the analog values present in the analog value storage means are converted into digital values by several A/D converters. It is believed that the use of several A/D converters has the advantage that the rate at which the analog value storage means has to be read out, is allowed to be slightly higher than with the previously explained embodiment. Then, in this case, various analog values have to be sent to various A/D converters. The A/D converters themselves can then in turn be operated at a lower conversion rate, so that conventional A/D converters can be used for this purpose.
According to another aspect of an exemplary embodiment of the present invention, a device for directly digitizing microwave signals reflected on the filling product surface of a filling product present in a receptacle is proposed, comprising: a reception circuit into which the reflected microwave signals have to be fed, and which is configured having a predetermined scanning rate for scanning same, an analog value storage means connected with said reception circuit for storing the analog values generated by said storage process at a storing rate corresponding to the scanning process, at least one A/D converter connected with said analog value storage means, which A/D converter converts the stored analog values into digital values, and a control means for controlling the reception circuit, the analog value storage means and the at least one A/D converter in such a manner that the microwave signals are scanned at the predetermined scanning rate, and are stored in the analog value storage means at a storing rate corresponding to said scanning rate, that the stored analog values are read out from the analog value storage means at a read-out rate lower than the storing rate and are converted into digital values in the A/D converter, and that the digital values are made available to an evaluation means for determining the filling level height.
According to still another aspect of an exemplary embodiment of the present invention, a device for directly digitizing microwave signals reflected on the filling product surface of a filling product present in a receptacle is proposed, comprising: a reception circuit into which the reflected microwave signals have to be fed, and which is configured having a predetermined scanning rate for scanning same, an analog value storage means connected with said reception circuit for storing the analog values generated by said storage process at a storing rate corresponding to the scanning process, at least one A/D converter connected with said analog value storage means, which A/D converter converts the stored analog values into digital values, and a control means. The control means controls the reception circuit, the analog value storage means, the at least one A/D converter, and the digital value storage means in such a manner that the microwave signals are scanned at the predetermined scanning rate, and are stored in the analog value storage means at a storing rate corresponding to said scanning rate. Moreover, it is hereby controlled that the stored analog values are read out from the analog value storage means at a read-out rate lower than the storing rate, and are converted into digital values in the A/D converter, and that the digital values are stored in the digital value storage means. The values hitherto stored in the digital value storage means are then available to an evaluation means for determining the filling level height.
One aspect of an exemplary embodiment of the present invention is based for the first time on the idea of no longer temporally xe2x80x9celongatingxe2x80x9d the received microwave signals by means of a mixer, but of digitizing temporally xe2x80x9cnon-elongatedxe2x80x9d microwave signals by means of one or several A/D converters. For being able to achieve scanning rates required for a sufficient measurement accuracy notwithstanding the non-performance of temporal elongation, and of being able to use a digital value storage means having usual commercial access times despite of that, the analog value storage means rapidly reading specific analog values but outputting same in a slower manner, is provided. Through this device, known as such as FISO system, scanning rates can be achieved as with prior art with time elongation (by means of analog components) in spite of the very high frequencies of the xe2x80x9cnon-elongatedxe2x80x9d microwave signals. As soon as the corresponding digital values are stored in a digital value storage means, the processing of the digital values occurs and, hence, the evaluation takes place in the conventional manner. This means that the very high frequencies due to the no longer present mixer of the signals to be digitized, and therewith the necessary very high scanning rates, are xe2x80x9cinterceptedxe2x80x9d by the use of a FISO system.
If only one A/D converter is used for digitizing the microwave signals or the therefrom determined envelope curve, then it is purposeful under certain circumstances to provide at least two buffers between the A/D converter and the final storage unit, into which buffers digital values are alternatively written in. Thereby, even with conventional memory components such as SRAM or DDR-SRAM having cycle times of, for example, 2.5-5 ns, the high frequency of the AID converter can be employed. The more buffers provided, the slower the memory modules are allowed to work.
Therewith, it is believed that it is possible according to an aspect of the present invention to perform a digitization with conventional modules despite the immensely high frequency of the input signals, a fact that eliminates the disadvantages of analog components, and in particular of the previous mixer.
By using a FISO system, it may also be possible for the first time to use hitherto usual xe2x80x9cslowxe2x80x9d A/D converters.
The exemplary embodiment of the device according to the present invention for the direct digitization according to the aforementioned explanations can be arranged as a single constructional unit spaced apart from the remaining components of a filling level measuring device. It is believed that it is, however, particularly purposeful to provide such devices or such a circuit directly within the filling level measuring device. Said device or said circuit may then preferably accommodated in the housing of a filling level measuring device. For the further explanation and the better understanding, several embodiments of the invention are described in more detail in the following with reference to the attached drawings.